Two rivers flowing at different levels in the same region i.e., on the ground surface and below the ground surface but following the same direction and gradient may be called twin-river system. Such river system is an example of geological unusual instance. Such twin-river system formed by the Amazon River and subterranean river just below the Amazon, named the Hamza River, has been recently discovered in the year 2011 in Brazil.
Valiya Mannathal Hamza, the Indian born scientist of National Observatory of Rio (Rio de Janeiro), Brazil, discovered the largest underground river below the Amazon River in Brazil in 2011. This subterranean river has been named after V.M. Hamza as Hamza River. Thus, such river system is called twin-river system.
The following are the characteristic features of the Hamza and the Amazon twin-river system:
1. The subterranean Hamza river flows for a total distance of 6000 km at a depth of about 4 km from the bed of the Amazon River and follows the same flow direction of the Amazon.
2. The Hamza river runs all the way from the foot hills region of the Andes Mountain to the Atlantic Ocean in a west-east direction. It drains into the Atlantic Ocean as does the Amazon River but at different levels.
3. It may be mentioned that except for flow direction the Amazon and the Hamza rivers greatly vary in terms of their width, flow velocity etc. as follows:
a. The width of the Amazon ranges from 1 km (1000m) to 100 km at different locations but the width of the Hamza River varies from 200 km to 400 km. It means the Hamza is much wider than the Amazon.
b. The average speed of water in the surface river Amazon is 5 m/s whereas it is less than one millimeter per second in the subterrean Hamza river. Thus, the Hamza is exceedingly sluggish in flow velocity.
4. The primary source of water of the Hamza river is infiltration of surface water through permeable sedimentary rocks. In fact, there are west-east trending faults which act as conduits for water to sink to greater depth and form subterranean stream (Hamza).
5. The general gradient of ground surface is from west to east i.e. from the foot hills of the Andes in the west to the Atlantic coast in the east. This surface gradient is also maintained in subsurface region and hence both surface stream Amazon and subsurface stream Hamza follow the same gradient i.e. from west to east.
6. In other words, west to east topographic gradient directs the water of the Amazon and the Hamza river just below the Amazon to adopt Atlantic-bound flow path.